Methods for the production of polyamide (PA) are well known (e.g., Kohan, Nylon Plastics Handbook, Carl Hanser Verlag, Munich, 1995 and Kunststoff Handbuch, 3. Technische Thermoplaste, 4. Polyamide, Carl Hanser Verlag, Munich, 1998 (pages 42-47 and 65-71)). According to such methods, in the first step, caprolactam is at least partially cleaved under the action of water to yield the corresponding aminocaproic acid, and in the subsequent step, further polymerized by polyaddition and polycondensation with the removal of water.
On an industrial scale, polyamide is produced in a VK tube (VK=simplified, continuous), in which liquid caprolactam is introduced, with approx. 1-4% of water, from the top into a single, vertical, tubular reactor or a series thereof. Excess water is removed by distillation.
Polymerization is performed at temperatures of between 240° C. and 270° C. in 15 to 30 hours. The process may be accelerated by a few hours by providing an upstream pressure stage, in which the rate-determining cleavage of caprolactam is performed under elevated pressure but otherwise similar conditions.
In this process, the water content of the melt determines the achievable viscosity. As a general rule, relative viscosities of around 2.6-3.0 (measured as a 1% solution in m-cresol at 25° C.) may be achieved.
For thermodynamic reasons, conversion in this process is limited. For example, at equilibrium at 270° C., in addition to polyamide, there is still a residual content of approx. 10% of low molecular weight species, substantially caprolactam and cyclic oligomers (dimer-tetramer). This residual content drops significantly as temperatures fall. Because the residual content is disruptive to subsequent applications, it is necessary to minimize the residual content. This may be achieved by aqueous extraction or by vacuum delactamization.
Viscosities higher than those stated above, and which are required for certain applications (for example extrusion), are conventionally achieved in a subsequent solid phase postcondensation at temperatures of 30-80° C. below the polymer melting point under a vacuum or a countercurrent stream of inert gas. For example, starting from polyamide 6 with a relative viscosity of 2.8, a relative viscosity of 3.8 is achieved in 24 hours at 185° C.
Alternatives to these tried and tested approaches have recently been described that permit distinctly faster polymerization, in particular faster melt postcondensation of (pressure stage) prepolymers, and which also make it possible to obtain higher viscosities directly in the melt.
WO-A 00/23501 and WO-A 00/23502 describe melt postcondensation of prepolymers in horizontal, tubular reactors with spreading over the surface, resulting in the production of large, self-renewing melt surfaces. A stream of inert gas is passed through the reactor to bring about more efficient dewatering of the melt by reducing the partial pressure of water in the gas phase. In this manner, relative viscosities of up to 4.0 have been achieved in residence times of 4.5 hours. The process described in the Examples therein was performed at 267° C. In addition, demonomerization was achieved in this process. In a preferred embodiment, in a first process stage caprolactam ring opening was also performed under the action of water in the gas phase (elevated H2O partial pressures) in a reactor with large, self-renewing surfaces.
EP-A 137 884 and U.S. Pat. No. 4,540,774 describe a related process for integrated demonomerization and postcondensation, wherein a more effective demonomerization may be achieved by operating the horizontal, tubular reactor under a vacuum (p<5 torr). In the Examples therein, the process was operated at 284° C.
DD-A 227 140 describes a multistage process for melt postcondensation of pressure stage prepolymer using a sequence including a melt drying stage and a subsequent polycondensation stage. Owing to the use of falling polymer threads of small diameter and a countercurrent stream of nitrogen, the polymer dries rapidly such that the water content in the melt is taken far from equilibrium. This brings about an elevated rate of polycondensation in the subsequent condensation stage. Additional water is liberated by the condensation, which is removed in a further melt drying stage, such that a rapid build-up of viscosity can be achieved in the next polycondensation reactor. In addition to using falling polymer threads to achieve large surface areas, a film evaporator is also described in which the melt flows as a thin film over a vertical metal gauze, also under a countercurrent stream of inert gas. In the Example therein, the described process is performed at 275° C.
A similar processing scheme is described in DE-A 19 506 407, in which large, self-renewing melt surfaces and dewatering are achieved under a countercurrent stream of inert gas on expanded metals, followed by a melt sump, which functions as do the above-described polycondensation reactors. These reactors are connected in series as a cascade. In this process, an attempt is made to ensure sump residence times of <0.5 h, while in the Example therein, the melt temperature is 280° C. Where three such reactors are used, a relative viscosity of 3.8 is achieved in an overall melt postcondensation time of 2 hours.
DD-A 234 430 describes a similar process, but without stating details of precise process parameters, using various degassing reactors, which degas thin films of melt with the assistance of vertical expanded metal/perforated sheets.
DE-A 69 512 437 describes a process in which a more rapid build-up in PA viscosity is achieved by mixing a stripping agent (N2) into the polymer melt under pressure and subsequently depressurizing the mixture to strip H2O out under a vacuum. The foaming seen on depressurization also results in large surface areas, and thus, effective dewatering of the melt. The melt is kept at the same, unstated temperature until equilibrium is established. The preferred embodiment of this process is to use an extruder.
As can be appreciated by those skilled in the art, the above-detailed processes have the following elements in common:    1. Effective dewatering of polyamide melts brings about very high rates of polycondensation due to the great distance from equilibrium.    2. The effective dewatering is achieved by increasing the surface area of the melt, thereby ensuring short diffusion paths for water and by reducing the partial pressure of water in the gas phase to further increase the efficiency of melt drying.    3. The temperatures described in the relevant Examples are above 265° C.
However, it is well-known by those skilled in the art that the above-stated conditions are precisely those under which secondary reactions occur which may bring about decarboxylation and branching. DE-A 22 55 674 accordingly describes, precisely in polyamide melts having a water content far below equilibrium, distinct decarboxylation and branch formation.
One attempt to overcome the disadvantages of the secondary reactions, which is described in DE-A 22 55 674, is to use an inert gas with an elevated water vapor partial pressure. However, the disclosed procedure has the disadvantage that the achievable viscosities are distinctly lower than where the process is performed without water vapor.